This invention relates to a method and apparatus for taking distance images. A distance image is the image of an object whose image spots do not, as in the case of a normal image, correspond to the lightintensities or colors of the object points, but rather to the respective distances of the corresponding object points.
A method is known from German Patent document DE 41 29 912 C1, for taking distance images in which a single light pulse is emitted. The light pulse reflected by the object is received on an image receiving unit which includes a shutter device connected in front of the unit. The transparency of the shutter device varies according to a predetermined time dependent function during the reception. Via at least two reflection images of the same distance range which are taken in this manner, the distance and the reflectivity of the object can be determined.
It is an object of the present invention to further improve the method described in German Patent document DE 41 29 912 C1 in such a manner that the signal-to-noise ratio will be increased.
This object is achieved by the method for producing distance images according to the present invention, in which a light signal is emitted and the (time-delayed) reflected light signal is received via a receiving device. At least two reflection images of the same distance range are taken, emitted light signal is intensity-modulated by a modulation frequency, and for at least one of the two reflection images, the sensitivity of the receiving device is modulated by the same modulation frequency and is locked in-phase with respect to the modulation frequency. Subsequently, by comparing of the light intensity values in the first and second reflection image, the reflectivity and the distance of the objects are determined.
As a further development of the method according to German Patent document DE 41 29 912 C1, in which a single light pulse is emitted, a phase method is now suggested in which continuous illumination is performed. In this case, a light source, such as an illuminating laser, is intensity-modulated in a continuous operating mode by means of a high-frequency drive voltage, i.e., CW-operation. By means of the same frequency, the sensitivity of an image receiving unit or receiving device is controlled. For example, the intensification of an image intensifier, i.e., channel plate, within the image receiving unit is modulated corresponding to the amplitude of the drive voltage.
When an object which is illuminated by a laser modulated by the same frequency is taken by the image intensifier, the photographed light intensity (half-tone values) of the individual image spots is a function of the reflectivity of the object and of the phase shift between the intensifying function of the image intensifier and the reflected laser light. The deciding factor is that this phase shift is dependent on the distance of the imaged objects. The modulation frequency of the laser illuminating unit and the image intensifier can be selected such that, in a distance interval which is to be imaged and measured, a phase shift of .pi. is achieved. This results from twice the light path which is covered from the start of the distance interval to the end. This means that there is a clear light intensity modulation in the image between 0 (minimal distance value within the distance interval to be measured) and the maximum (maximal distance value within the distance interval to be measured). The distance ranges in front of and behind the image intensifier receive measured values corresponding to their distance. These values are basically identical to the values within the measuring interval so that ambiguities will arise. By the variation of the frequencies and/or the suitable adaptation of the measuring intervals, such ambiguities can be avoided. When the measuring frequency is increased, the measuring ranges which are repeated by modulo .pi., that is, the ambiguities, become more frequent. However, the measuring precision also becomes higher, so that a compromise must be found here or multiple measurements must be used.
For precise distance measuring, the reflectivity of the object to be measured must also be determined. This may be achieved by taking another reflection image, in which however, the intensification of the image intensifier is not modulated, but is kept constant. As a result, a laser-illuminated, intensity-modulated image of the imaged scene is obtained.
As an alternative, when the second reflection image is taken, the phase of the modulation frequency may also be shifted relative to the first reflection image. The two images contain all information concerning the distance and the reflectivity of the objects. By means of simple mathematical algorithms, these two values can be calculated from the half-tone values of the individual image spots of the two images.
As required, the determined values for the distance and the reflectivity of the objects may be displayed on a monitor in false-color coding.
In order to control the sensitivity of the receiving device, shutter devices may be used instead of the above-mentioned image intensifier.
The two reflection images may be taken either at the same time (in several receiving devices), or sequentially.
In comparison to the alternative embodiment in German Patent document DE 41 29 912 C1, the method according to the present invention has the advantage that a higher measuring precision can be achieved (resolution in the cm to mm range). In addition, as a result of the continuous illumination, the signal-to-noise ratio is significantly improved.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.